In ultrasound imaging, spatial resolution and depth penetration are important parameters to quantitatively assess image quality. Generally, the wavelength at the ultrasound center frequency determines spatial resolution, with revolution improving as the frequency increases. However, tissue attenuation typically increases as a linear function of frequency, even becoming nonlinear at higher frequencies (e.g., greater than 40 MHz) commonly employed in intravascular ultrasound imaging applications. Although high frequency ultrasound facilitates high resolution imaging in the near field, the depth penetration may be compromised, rendering the deep tissue structures difficult to identify.
To enhance the depth penetration without the loss of spatial resolution, transmit voltage levels are typically increased to deliver more transmit energy to the imaging target. However, this approach is limited both by regulations of diagnostic medical ultrasound and by the nature of the intravascular ultrasound imaging environment, such as the long cable length between ultrasound transducer elements and imaging system electronics and the small size of electronics integrated with the catheter mounted transducer.
Rather than increasing the transmit voltage levels, coded excitation methods using elongated modulated transmit bursts have been employed to address the dilemma of maintaining depth of penetration while increasing operating frequency for improved spatial resolution. This method is called pulse compression. An ultrasound imaging apparatus using pulse compression employs a coded long pulse instead of the conventional short pulse. One type of coded excitation methods uses Golay codes. A Golay code is a binary code modulated with a short burst. Two different binary code (of particular sequence) constitute a Golay pair. When two codes are separately decoded and summed, range sidelobes are completely eliminated with only the main lobe remaining. Due to that characteristic of Golay codes, there have been great endeavors to take advantage of Golay codes in ultrasound imaging apparatuses.
In practice, the range sidelobes are not completely removed, due to nonlinear propagation of ultrasound within tissue structures, motion artifacts, and other non-idealities. Particularly for rotational intravascular ultrasound, the continuous rotation causes a slight angular misalignment for adjacent A-scans used to produce a Golay pair, resulting in increased range sidelobe levels.